This invention relates to testing of electronic circuits and, more particularly, to electro-optic techniques for making such measurements.
The need for non-invasive testing of integrated circuits and devices is rapidly becoming more apparent. The increasing speed and complexity of current and future circuitry has already exceeded the capabilities of conventional allelectronic testing techniques. In recent years, picosecond laser systems have been applied to a variety of optically based measurement schemes for making electrical waveform measurements at internal circuit nodes. However, some techniques are restricted by being applicable to only a particular material system or by the necessity of operation in vacuum.
External electro-optic (e-o) probing techniques exploit the fact that two-dimensional circuits have an open electrode structure which gives rise to fringing fields above the surface of the circuit. See, for example, J. Nees et al, Electron Lett., Vol. 22, No. 17, p. 318 (1986) and G. Mourou an J. A. Valdmanis (this applicant), U.S. Pat. No. 4,618,819 issued on Oct. 21, 1986. Dipping a minute e-o crystal probe into these fields changes the crystal's birefringence, which can then be measured optically; that is, by a pulsed light beam which is passed through the e-o crystal and is reflected from the circuit. The advantage of such external probes is that they can be applied to almost any type circuit, because the interaction is based on a field effect. Since no charge is removed from the circuit, the probe does not need to make electrical contact to the circuit.
All external probes to date have relied on the transverse e-o effect in lithium tantalate. Lithium tantalate is a well-known e-o material that exhibits a relative large e-o effect, but only in the transverse geometry. In this geometry, the optic axis of the e-o material is transverse to the direction of the light beam and parallel to the surface of the circuit. Consequently, the e-o probes are sensitive only to fields parallel to the surface of the circuit. These transverse fields exist mainly between conductors on the surface which are at different potentials. In order to measure the signal on a particular conductor, an AC ground has to be provided on an adjacent conductor. The probe is then centered between these two conductors. But, since two-dimensional circuits have all their conductors in the same plane, there can be many overlapping transverse fields at any one point on the surface of the circuit, leading to crosstalk at the probing point. Sensitivity of the probes also depends on conductor spacings and widths, since both parameters affect transverse field strength. In practice, these effects can be avoided only by careful electrode layout. In addition, because the fringing field strength decreases rapidly with height above a conductor, the probe-to-conductor spacing has to be carefully controlled, rendering the transverse e-o technique unsuitable for many applications.
In contrast, longitudinal e-o probing has not been utilized in external probes, but has been demonstrated in GaAs circuits, where the GaAs substrate itself is electro-optic. Because the substrate is part of the circuit under test, this technique will be referred to as "internal" e-o probing. As described by J. L. Freeman et al, Appl. Phys. Lett., Vol. 47, No. 10, p. 1083 (1985), this technique involves directing the light beam through the backside of the substrate and reflecting it from conductors on the front side. Electric fields generated in the substrate induce birefringence which is measured by the light beam. Thus, this technique cannot be used to test integrated circuits formed on non-electro-optic substrates; e.g., the entire class of silicon ICs on silicon substrates is excluded, as are hybrid ICs such as GaAs ICs on silicon substrates, circuits on ceramic substrates, and printed circuit boards.